Vanille – Sentinel Crystarium

Character Vanille
Role Sentinel
Vanille as a Sentinel? Surely not! And truth be told, she's not very good at it. She has Mediguard at least, but not much else, including Challenge. She does have more HP than Lightning or Hope, but both of them are evasion Sentinels anyway. Basically, you don't want Vanille to be your main Sentinel. She's fine in the Tortoise (SEN/SEN/SEN) paradigm, but not much else.
Compare Vanille – Sentinel Crystarium to other Crystariums

Related

Vanille

Eidolon Hecatoncheir (Chapter 11)
Base HP 350
Maximum HP 21,050
Base Strength 23
Maximum Strength 1,400
Base Magic 35
Maximum Magic 1,700
Starting Weapon Binding Rod
Primary Roles Ravager, Saboteur, Medic
Accessory Crystal Saboteur 5
Accessory Crystal Medic 8
Accessory Crystal Ravager 10
ATB Level Crystal Medic 8
Chapter 1

Fang – Sentinel Crystarium

Character Fang
Role Sentinel
Like Snow, Fang has access to every standard Sentinel Ability (i.e., except Elude), though her lower HP total and relative lack of defensive Weapons makes her a bit less effective than him. She does get Mediguard first, so I guess that's something.

Hope – Sentinel Crystarium

Character Hope
Role Sentinel
Despite having the lowest HP in the game, Hope makes for a fairly interesting Sentinel. Like Lightning, he doesn't get Challenge and favors Elude over Mediguard and Steelguard, but Hope also gets the counterattack Abilities, Entrench and Vendetta. He's fun to play around with, but not much use against truly powerful enemies.

Lightning – Sentinel Crystarium

Character Lightning
Role Sentinel
Lightning is an interesting Sentinel, relying on the Elude Ability rather than Mediguard or Steelguard. This Ability (the only one she can use besides Provoke) allows her to dodge attacks with a very high rate of success. However, it only works on Physical attacks, making Lightning a sitting duck against Magic-wielding foes. Still, when it works, it's very effective and requires very little healing.

Sazh – Sentinel Crystarium

Character Sazh
Role Sentinel
Sazh is all about defense as a Sentinel. That may seem to go without saying, but he doesn't get any evasion Abilities, and while he does get Vendetta he does not get Entrench or the Counter auto Ability. He also doesn't get Mediguard, so if Sazh is your defender, you'll need to keep an eye on his HP. He does have the second-highest HP total in the game, though, and is the only non-primary Sentinel with Challenge.

Snow – Sentinel Crystarium

Character Snow
Role Sentinel
Snow is the prototypical Sentinel, gaining every Ability except Elude. Snow's job is to stand there and take punishment. While he has the same Ability list as Fang, Snow is a bit better as a Sentinel because of his high HP total and access to Weapons with the Improved Guard and Improved Ward abilities.

Vanille – Commando Crystarium

Character Vanille
Role Commando
Vanille is the most limited Commando in the game (and has the silliest Weapon), lacking both Launch and Blitz. She does get Blindside and Faultsiphon, auto Abilities that are easy to take advantage of. However, her Role should generally be limited to that of a secondary Commando. She's more useful for the Augmented Attack effect than actual Commando Abilities.

Vanille – Medic Crystarium

Character Vanille
Role Medic
Vanille and Hope are the only Characters with access to every Medic Ability, and while Hope's magic stat is naturally higher, Vanille is plenty effective as a Medic as well. She and Hope are the only Characters who can use the amazing Curaja Ability, a vital healing spell in the endgame.

Vanille – Ravager Crystarium

Character Vanille
Role Ravager
Vanille is a good Ravager with a number of holes in her Ability list. Like Hope, she lacks any -strike Abilities, making her unable to damage enemies that are immune to Magic. She can still advance their chain bonus (and doesn't need to worry about strength!), but this can be a big problem against certain foes. She also lacks Aeroga, Thundaga, and Waterga, and doesn't get the Vigor Ability. None of this is crippling, but it does set Vanille firmly behind Hope as far as Ravagers go.

Vanille – Saboteur Crystarium

Character Vanille
Role Saboteur
Vanille is a very effective Saboteur with a variety of Abilities, though she is more concentrated on defense-lowering debuffs. She cannot inflict Slow, Curse, or Daze, and also lacks Fogga and Painga, but has single- and multi-target versions of every other Saboteur Ability. Beyond that, she has Death, a unique Ability that either just kills a target or deals a large amount of damage to them. Since very few non-boss enemies are actually immune to Death, you can get cheap, fast kills against a number of difficult foes (up to and including Adamantoises) with it.

Vanille – Synergist Crystarium

Character Vanille
Role Synergist
Along with Fang, Vanille is one of two Characters that has access to Bravera, Faithra, Protectra, and Shellra, high-powered Abilities that have a short duration to balance their extra effectiveness. Unlike Fang, Vanille has a decent number of standard Synergist Abilities as well, including Veil, Vigilance, and the Bar-spells. She is the only Character in the game that doesn't get Haste, however, so she's not very effective as your only Synergist.

Sentinel

Bonus Type Damage Reduction

Sentinel

Abbreviation SEN
Role Bonus Damage Reduction
Base Bonus (Self) Damage Resistance: +40%, Wound Resistance: +30%
Role Level I Bonus (Self) Damage Resistance: +45%, Wound Resistance: +33%
Role Level II Bonus (Self) Damage Resistance: +50%, Wound Resistance: +36%
Base Bonus (Allies) Damage Resistance: +7%, Wound Resistance: +5%
Role Level II Bonus (Allies) Damage Resistance: +9%, Wound Resistance: +7%
Role Accessory Defender's Topaz (Grants Damage Resistance: +3%, Wound Resistance: +3% to self)